1. Field of the Invention
The present invention relates generally to micro electromechanical systems (MEMS) and, more particularly, to circuits and methods for controlling a voltage in MEMS cells.
2. Description of the Background Art
A MEMS or micro electromechanical (MEM) device typically includes micromechanical structures or light modulating elements that may be actuated using electrical signals. The light-modulating elements may comprise, for example, Grating Light Valve™ (GLV™) light modulating elements available from Silicon Light Machines, Sunnyvale, Calif. (GLV™ and Grating Light Valve™ are trademarks of Silicon Light Machines). A light modulator may include an array of moveable structures referred to as “ribbons.” Light modulators may be used in various applications, including video, printing, optical switching, and maskless lithography, as just a few general examples.
Lithography and imaging applications using MEMS devices often require very large numbers of pixels to achieve high data rates. In some applications, a two-dimensional array of pixels may be required and the total number of pixels is typically too high to allow for individual MEMS cell driving control. That is, for a large two-dimensional array, each MEMS cell typically does not have its own dedicated driver circuit. Instead, row drivers, including driver circuits for each row, and column, including access circuits for each column, are used to control the MEMS cells. Such an arrangement 100 is shown in FIG. 1. The row drivers 102 deliver the stimulation signal for a given row of elements and the column drivers 104 are used to select the correct row stimulation for a given column. By using both together, all elements (MEMS cells 106) may be driven with the desired stimulation in turn.
The MEMS cells 106 can include light modulators organized as “pixels.” These pixels can require precise analog control for providing gray scale at an image target. In some applications, these MEMS arrays use pulsed laser sources on the surface of the modulator. Accordingly, the MEMS device would only need to be at an intended voltage state for a relatively short period of time.
FIG. 2 is a schematic diagram of a conventional MEMS driver circuit 200 that can be used with the array 100 shown in FIG. 1. This example uses a sample and hold circuit at each cell with sampling of the row analog data at each column in turn. Typically, this approach would require a high voltage sample and hold or a low voltage one with an amplifier on the output stage. In FIG. 2, a digital-to-analog (DAC) 202 drives a signal to a receiver 204 on the MEMS die 201. The output of the receiver 204 can be the row drive signal 206 and it can connect to a column select switch 208. Capacitor C1 can connect to the output of the column switch 208 and to the input of amplifier OP 210. The output of OP 210 can connect to a transfer switch 212. Capacitor C2 can connect to the transfer switch 212 and to a high voltage (HV) driver section. The HV driver section can include a resistor R2 connected between a high-voltage supply (HV supply) 214 and the MEMS cell 106 via the MEMS node 216. Transistors Q1 and Q2, along with resistor R3 and voltage source V1 can be used to generate the current needed to produce a voltage across R2 complete the HV driver section. The circuit 200 allows for a voltage range of between about 3V and about 18V at the MEMS node 216 of the cell 106.